Experimental and Theoretical Insights into the Effect of Dioldibenzoate Isomers on the Performance of Polypropylene Catalysts

Experimental investigations and density functional theory (DFT) calculations were carried out to study the comprehensive effect of different 3,5-heptanedioldibenzoate (HDDB) optical isomers as the internal electron donor on the catalytic performance of Ziegler−Natta catalysts. The experimental catalytic activity of HDDB has a positive correlation with the relative content of the mesomer incorporated during catalyst preparation, while the hydrogen response of HDDB displayed a negative correlation with the relative content of the mesomer. In order to apply the DFT calculation results to the macroscopic activity of the catalyst, the content of the active centers of the catalyst was analyzed. Assuming that the content of the active centers is proportional to the internal electron donor content of the catalyst, binary linear regression was carried out, which showed a good linear correlation between experimental activity data and internal electron donor content. Furthermore, the fitted activity of the single active centers aligned well with the calculated activation energies. These results revealed that the catalytic activity of polypropylene (PP) catalysts is dependent on both the active center content and the catalytic activity of an individual active center. Additionally, the lower hydrogen response of HDDB leads to a higher molecular weight of polypropylene obtained from the RS-containing catalyst compared to the SS-containing catalyst. Further study reveals that the hydrogen transfer reactions of 2,4-pentanediol dibenzoate (PDDB)/HDDB are influenced by the orientation of the methyl/ethyl groups in different isomers, which affect the activation energy differences between the hydrogen transfer reaction and the propylene insertion reaction, and finally influence the molecular weight of PP.


Introduction
Ziegler-Natta (ZN) catalysts have achieved great success in the polyolefin industry since their invention [1,2], with an annual production exceeding 150 million tons [3].Notably, the production of polypropylene (PP) reached 102.8 million tons in 2021, and more than 90% of polypropylene was produced using ZN catalysts.The ZN catalysts consist of TiCl 4 , MgCl 2 support, and internal electron donors (IDs), while external electron donors (EDs) and initiators are added during polymerization [4].Until now, industrial ZN catalysts have been developed to the fifth generation.Starting from the third generation of ZN catalysts, catalytic performance has been improved predominantly through the development of IDs [5].The IDs for the fifth generation of industrial ZN catalysts are nonphthalate compounds including 1,3-diethers [6,7], succinates [8,9], and diol diesters [10][11][12].
In addition, new electron donors such as sulfonyl amine [13][14][15] and its derived diesters [16] are also expected to be introduced into industrialization.
Despite the remarkable success and advancements of ZN catalysts in the industry, academic research on the mechanism of ZN catalysts is complicated, due to its multicomponent disperse structures and the challenges associated with the microstructure characterizations.Recently, several studies involving the exploration of microscopic active centers have been carried out [17,18].Fan et al. explored the catalyst active center numbers during polymerization reactions using the quenching method [19,20].Christophe and co-workers investigated the interactions between the support and the electron donor [21].Unfortunately, experiments on characterizing the precise structures of active centers are generally expensive and difficult.Since the 21st century, a large number of theoretical studies have been applied to the mechanism research of ZN catalysts [22], including explorations on the interactions among each component [23][24][25][26][27][28][29][30][31][32][33][34][35] and the influences of electron donors on the catalytic performance of ZN catalysts [36][37][38][39][40][41][42].Although earlier computational studies had inconsistent conclusions with the experiment results due to different models and computational methods [43], the polymerization process of propylene in the MgCl 2 /TiCl 4 /ID system has been gradually elucidated in recent years.During this process, TiCl 4 is adsorbed on a (110)-facet of MgCl 2 to form a stable active center, and the electron donor close to TiCl 4 enhances the stereoselectivity of the ZN catalyst.Some studies also revealed that triethylaluminium could affect the stereoselectivity of ZN catalysts [44][45][46].
Compared with the current qualitative research on ZN catalysts that explains the basic mechanism of ZN catalyst systems [47], there is a lack of quantitative simulation for industrial ZN catalysts.Unlike studies on isotacticity, which are easily explored through the comparison of different active energies for a single active center, research on macroscopic properties such as catalytic activity and average molecular weight is rare.Macroscopic property research must consider factors such as the quantity of active centers, various reaction conditions, and changes in the actual reaction environment.Moreover, significant differences in the actual reaction performance would appear when using different preparation methods for the same catalyst composition, even when using the same type of electron donor isomers [43].
Catalysts [11,12,48] containing different isomers of diol diester electron donors have significantly different properties.In our previous study [49], we reported that the ZN catalyst using optical isomers of electron donor PDDB (2,4-pentanedioldibenzoate) has significant differences in its ID content and stereoselectivity.DFT calculations showed that the primary factors contributing to the different content of ID isomers in the catalyst are the distinct conformations observed in the free state and adsorption state of ID molecules, along with different adsorption energies on the MgCl 2 support.
In this manuscript, we extend the exploration of diol diester IDs from PDDB to HDDB.Apart from the investigations on stereoselectivity, comprehensive theoretical calculations were carried out to elucidate the molecular level understanding of propylene polymerization catalyzed by ZN catalysts, and to investigate the factors contributing to diverse performances including catalytic activity, molecular weight, and other properties.

Materials
All operations were executed under a nitrogen atmosphere using glove-box and standard Schlenk (Beijing, China) techniques.Propylene and MgCl 2 were obtained from Sinopec (Beijing, China); AlEt 3 , TiCl 4 , and cyclohexylmethyldimethoxylalkoxysilane (CHMMS) were purchased from J&K Scientific (Beijing, China) without further purification; AlEt 3 and CHMMS were diluted in n-hexane to 0.5 mol/L and 0.1 mol/L, respectively.All other solvents were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and dried with 4Å molecular sieves in advance.

Catalyst Preparation
The stereoisomers of HDDB (characterized using 1 H NMR, see Figures S1 and S2) and the MgCl 2 -supported catalysts [11,12,48] were synthesized or prepared according to the previously reported procedure.A typical synthesis procedure of MgCl 2 -supported catalysts is as follows: First, 4.8 g magnesium chloride, 95 mL toluene, 4 mL epoxy chloropropane, and 12.5 mL tributyl phosphate were added successively to a nitrogen-purged five-necked reactor.The mixture was heated to 60 • C with stirring and maintained at this temperature for 1.5 h.Then, 1.4 g phthalic anhydride was added to the reaction mixture and the reaction was maintained for 1 h.The solution was then cooled down, and 56 mL TiCl 4 was introduced slowly below −20 • C.After the mixture was heated to 80 • C, 5 mmol of IDs (HDDB with different contents of mesomer and racemate, or others) was added and maintained for 1 h.After removing the supernatant, the residue was washed with toluene.The obtained solid precipitate was treated twice with 40 mL TiCl 4 and 60 mL toluene at 100 • C for 2 h.After sedimentation and filtration, the residue was washed with 60 mL hexane to obtain the solid catalyst.

Bulk Polymerization Process
Propylene polymerization was carried out in a 5 L stainless steel reactor with liquefied propylene [11,12,48].The typical procedure for the bulk polymerization of propylene was as follows: A total of 5 mL AlEt 3 , 1 mL CHMMS, about 9 mg of the solid catalyst, and 1.2 SL (standard liter) hydrogen were added.Then, 2.3 L liquid propylene was added to the 5 L stainless steel reactor, which had been replaced with propylene gas completely.The reactor was heated to 70 • C and the polymerization was processed at 70 • C for 1 h.Then, polypropylene (PP) resin powder was obtained after reducing both the temperature and pressure.

Characterization
Nuclear magnetic resonance ( 1 H NMR) was characterized using the Bruke dmx300 nuclear magnetic resonance spectrometer (300 MHz, solvent is CDCl 3 , TMS as internal standard, and measuring temperature is 300 K).
The content of meso-/rac-HDDB was detected using liquid chromatography (LC).The content of meso-/rac-HDDB in compounds was detected directly.The catalyst was first dissolved in an acidic media and then extracted using ethyl acetate, to be examined.Furthermore, separations were performed on the Waters-600E UPLC H-Class (Waters Corporation, 34 Maple Street, Milford, MA, USA) with a column of ACQUITY UPLC BEH C18 (50 mm × 2.1 mm, 1.7 µm).The absolute content of total HDDB (including mesomer and racemate) in the catalyst was calculated by using a standard curve.
The isotactic index (I.I) of the polymer was calculated using the following method: First, 2 g dried polymer sample is extracted with boiling heptane in an extractor for 6 h, then the residual substance is dried to a constant weight, and the ratio of the weight (g) of residual polymer to two is named as the isotactic index.
The melt flow rate (MFR) of the polymer was measured using the test standard GB/T 3682.1-2018 [50] (corresponds to the standard of ASTM D 1238-13).The MFR is the amount of the polymer (g) flowing through the capillary for 10 min, under a pressure of 2.16 kg at 230 • C.
The molecular weight and its distribution (PD) of the polypropylene were determined using a Waters Alliance V2000 gel permeation chromatograph (GPC) equipped with a refractive index detector, using three Polymer Laboratory MIXED-B columns and 1,2,4-trichlorobenzene as the solvent at 150 • C. The number-average and weight-average molecular weight (M n and M w , respectively) values were evaluated with reference to a polystyrene standard calibration.

Computational Details
All DFT calculations were performed with the Gaussian16 program, Revision C.01.All geometry optimizations of intermediates and transition states were carried out using PBE0-D3/6-311G(d,p) [51,52] in combination with the Becke-Johnson (BJ) damping correlation [53].Frequency calculations were also conducted at the same level of theory to obtain vibrational frequencies to determine the identity of stationary points as intermediates (no imaginary frequencies) or transition states (only one imaginary frequency).To avoid the errors associated with low frequency, Grimme's quasi-RRHO [54] model was used for Gibbs free energy calculations using the Sherm 2.3 program [55].Compared with the default RRHO model provided by Gaussian16, the quasi-RRHO model demonstrated a quantitative accuracy improvement.Gibbs free energies were calculated at 343.15 K and 1 atm for the stable structures of the catalyst and transition state structures.The MgCl 2 cluster model, derived from δ-MgCl 2 crystals, underwent geometry optimization with the Mg atoms constrained at a fixed distance of 3.6363 Å (Mg•••Mg) while the other atoms are free.

The Polymerization Experiments
The polymerization results of the ZN catalysts with different HDDB stereoisomers as IDs are shown in Table 1 (see Table S1 for the polymerization results of PDDB).With the increase in mesomer content (see Figures S3-S16), the ID content, the activity (AC), and stereoselectivity (characterized using the isotactic index, I.I.) of the catalysts, as well as the molecular weight of the polymer product shows an increasing trend, while the hydrogen response (characterized using the melt flow rate, MFR) and the molecular weight distribution (characterized using the polydispersity, PD) exhibit a decreasing or narrowing trend.These results suggest that the stereoisomer content of HDDB in the catalysts plays a crucial role in the catalytic performance during propylene polymerization.

The Active Center Models for DFT Calculations
HDDB has two unequal stereoisomers: racemate and mesomer.In the free state, the free energy of the mesomer is 1.5 kcal/mol higher than that of the racemate isomers.Two oxygen atoms of carbonyl groups in the RS-isomer are in the same direction, while those of SS-isomers are in the opposite direction (See Figure 1 for the structure of free HDDB).Based on our previous work [49], where we compared th PDDB on MgCl2 and the relative energy of various active center m the trans-a model (differing only in left/right order as trans-a/b) re ter structure with the lowest energy for both RS-PDDB and SS-P the structure for the active center with the lowest energy and stere lated, respectively, revealing that the configuration of donor H PDDB (Table 2).Similarly to PDDB, the key difference in the transi of insertion reactions with the RS-HDDB or SS-HDDB isomer is th group.For the RS isomer, the ethyl group pointed towards the Ti a the space near the Ti center, resulting in a large energy difference i reactions of propylene molecules.Conversely, for the SS isomer, t perpendicular to the (110)-facet of MgCl2, which has less influen resulting in less energy difference for the insertion reaction of pro uscript, we maintain the use of the trans-a model as the active cent Figure 2 for the active center structure of HDDB).Based on our previous work [49], where we compared the adsorption energy of PDDB on MgCl 2 and the relative energy of various active center models, we assumed that the trans-a model (differing only in left/right order as trans-a/b) represents the active center structure with the lowest energy for both RS-PDDB and SS-PDDB.With this model, the structure for the active center with the lowest energy and stereoselectivity were calculated, respectively, revealing that the configuration of donor HDDB resembles that of PDDB (Table 2).Similarly to PDDB, the key difference in the transition state (TS) structures of insertion reactions with the RS-HDDB or SS-HDDB isomer is the orientation of the ethyl group.For the RS isomer, the ethyl group pointed towards the Ti atom, thereby restricting the space near the Ti center, resulting in a large energy difference in the siand re-insertion reactions of propylene molecules.Conversely, for the SS isomer, the ethyl group is nearly perpendicular to the (110)-facet of MgCl 2 , which has less influence on the active center, resulting in less energy difference for the insertion reaction of propylene.So, in this manuscript, we maintain the use of the trans-a model as the active center model for HDDB (See Figure 2 for the active center structure of HDDB).Based on our previous work [49], where we compared the adsorption ene PDDB on MgCl2 and the relative energy of various active center models, we assume the trans-a model (differing only in left/right order as trans-a/b) represents the activ ter structure with the lowest energy for both RS-PDDB and SS-PDDB.With this m the structure for the active center with the lowest energy and stereoselectivity were lated, respectively, revealing that the configuration of donor HDDB resembles t PDDB (Table 2).Similarly to PDDB, the key difference in the transition state (TS) stru of insertion reactions with the RS-HDDB or SS-HDDB isomer is the orientation of th group.For the RS isomer, the ethyl group pointed towards the Ti atom, thereby rest the space near the Ti center, resulting in a large energy difference in the si-and re-ins reactions of propylene molecules.Conversely, for the SS isomer, the ethyl group is perpendicular to the (110)-facet of MgCl2, which has less influence on the active resulting in less energy difference for the insertion reaction of propylene.So, in thi uscript, we maintain the use of the trans-a model as the active center model for HDD Figure 2 for the active center structure of HDDB).

DFT Calculations about the Catalytic Activity
Considering that the transition state energy for the si-insertion reaction is lower than that of re-insertion (see Table 1), the si-insertion and propylene-catalyst complex were selected as the reaction channel and energy starting point, respectively to calculate the energy barriers for the active center with a trans-a structure.The structures for the HDDB-catalyst complex and HDDB-catalyst-propylene (si-insertion) complex are listed in Figures 3 and 4, respectively.According to the calculation results on activation energies, the activity of SS-HDDB is slightly higher than that of RS-HDDB (see Table 3).The reason for the difference can be seen in Figure 4.The hydrogen atoms of RS-HDDB are only 1.95 angstroms apart from the hydrogen atoms of the ethyl group representing the polymer chain.This shows a strong steric effect.The same distance for SS-HDDB is 2.49 angstroms, and the interference with the reaction center is almost negligible.

DFT Calculations about the Catalytic Activity
Considering that the transition state energy for the si-insertion reaction is lowe that of re-insertion (see Table 1), the si-insertion and propylene-catalyst complex w lected as the reaction channel and energy starting point, respectively to calculate t ergy barriers for the active center with a trans-a structure.The structures for the H catalyst complex and HDDB-catalyst-propylene (si-insertion) complex are listed in 3 and Figure 4, respectively.According to the calculation results on activation en the activity of SS-HDDB is slightly higher than that of RS-HDDB (see Table 3).The for the difference can be seen in Figure 4.The hydrogen atoms of RS-HDDB are on angstroms apart from the hydrogen atoms of the ethyl group representing the po chain.This shows a strong steric effect.The same distance for SS-HDDB is 2.49 angs and the interference with the reaction center is almost negligible.

Analysis of Factors Affecting Catalytic Activity
According to the experimental data from Table 1 and Table S1, it is evident that the catalytic activity of the catalyst increases with a higher RS isomer content, while the overall IDs would also be increased.We suggest that the adsorption modes for different ID molecules on the MgCl 2 support are similar during the catalyst preparation processes.Assuming an ideal single-layer adsorption model, where the number of active centers is proportional to the quantity of ID molecules, the model for catalytic activity can be described as Here, A Tmacro represents the total activity of the catalyst, A RSmacro is the macroscopic activity of the active centers with the RS isomer, and A SSmacro is the macroscopic activity of the active centers with the SS isomer.A RS is proportional to the activity of the single RS isomer active center, C RS is the relative content of the RS isomer in the catalyst, A SS is proportional to the activity of the single SS isomer active center, and C SS is the relative content of the SS isomer in the catalyst.Additionally, we defined the relative activity ratio of the single SS isomer active center to that of the RS isomer as A SS /A RS .
The macroscopic activity data and ID isomer content, as presented in Table 1 and Table S1, underwent binary regression analysis and the findings are summarized in Table 4.The outcomes in Table 4 indicate that the catalytic activity for the active center with the SS isomer is slightly higher than that of the RS isomer for both PDDB and HDDB, which is consistent with the calculated results.Figure 5 shows the changes in macroscopic activity attributed to the active centers when the isomer content increased for both the RS and SS isomers.Notably, the depicted relationship demonstrates a high degree of linearity, with a regression coefficient exceeding 0.999.The calculation results indicated that the activity for the active center with the RS isomer is lower than that of the SS isomer, while the macroscopic activity for the catalyst exhibits a positive correlation with the RS content in the experiments.These results revealed that the mesomer of dioldibenzoate improves the experimental catalytic activity of polypropylene (PP) catalysts through increasing the active center content, rather than the catalytic activity of individual active centers.This phenomenon can be attributed to the fact that it is easier for the RS isomer to be adsorbed on MgCl2 surfaces (see Table 5), resulting in an increase in the content of active centers.This conclusion is also consistent The calculation results indicated that the activity for the active center with the RS isomer is lower than that of the SS isomer, while the macroscopic activity for the catalyst exhibits a positive correlation with the RS content in the experiments.These results revealed that the mesomer of dioldibenzoate improves the experimental catalytic activity of polypropylene (PP) catalysts through increasing the active center content, rather than the catalytic activity of individual active centers.This phenomenon can be attributed to the fact that it is easier for the RS isomer to be adsorbed on MgCl 2 surfaces (see Table 5), resulting in an increase in the content of active centers.This conclusion is also consistent with our previous work [49], where the mesomer ID molecule is easier to be adsorbed on the (110)facet of the MgCl 2 support with TiCl 4 , compared with the racemate isomer.For the ratio of TiCl 4 /ID and the catalyst preparation, the procedure remains unchanged in the series of experiments (Entry 1 to Entry 7 in Table 1), and the active center content is reasonably positively proportional to the ID content.The result for the binary regression analysis in Table 4 and Figure 5 is consistent with the calculated relative activity for the active centers with the SS/RS isomer in Table 3, which reveals that the assumption above is suitable for our catalyst system.These results revealed that the catalytic activity of polypropylene (PP) catalysts is dependent on both the active center content and the catalytic activity of the individual active center.a The data of PDDB have little difference to reference [49] because of the different models of processing frequency used in the Gibbs free energy calculation (calculated at 298.15 K and 1 atm).

Hydrogen Transfer Reaction
The chain transfer reaction is quite important in the polymerization process, and the relative rate of the hydrogen transfer reaction to the polymerization affects the molecular weight of polypropylene.Previous studies suggest that the chain transfer to the monomer serves as the primary chain transfer reaction, which could be applied in heterogeneous ZN catalysts [56,57], similar to metallocene [58,59] and non-metallocene systems [60,61].Under conditions where the monomer concentration is low or the alkylaluminium concentration is relatively high, the chain transfer to the metal and the chain transfer to the cocatalyst may become the dominant chain transfer reactions [62].In the area of industrial ZN catalysts, the use of hydrogen to control molecular weight makes hydrogenolysis (i.e., the hydrogen transfer reaction) the primary chain transfer reaction [56,63].The hydrogen transfer reaction can be represented by Equation (2): Considering the existence of H 2 , neglecting other chain transfer reactions (chain transfer to monomer, chain transfer to alkylaluminum, and ethyl β-dehydrogenation) and assuming constant concentrations of each reactant, the relationship between the hydrogen transfer reaction and the molecular weight can be expressed as follows: where M w is molecular weight, k p is the rate of propylene insertion reaction, C p is the concentration of propylene, k H is the rate of hydrogen transfer reaction, C H is the concentration of hydrogen, and ∆G H and ∆G p are the activation energies of the hydrogen transfer reaction and the propylene insertion reaction, respectively.
The hydrogen transfer reaction and propylene insertion reaction compete for the same active center.The difference between their activation energies is positively correlated to the molecular weight of polypropylene.Substituting the propylene molecule with an H 2 molecule in the model of Figure 4 allowed for the calculation of the hydrogen transfer reaction.The results from the Gibbs energy calculations revealed that there is no stable intermediate complex for H 2 and the active center.Instead, the hydrogen transfer reaction proceeded directly from a separated state to a transition state.In contrast to Ref. [33], the transfer of polymer chains (represented by the ethyl group in Figure 6) with H 2 was attributed to the interaction between the β-H and Ti center, while the insertion reaction of propylene was attributed to the interaction between the α-H and Ti center (see Figure 6 for the transition state structures).The activation energies are listed in Table 6, which suggests that the transfer activation energy of H 2 at the RS active center is higher than that of the SS active center for both PDDB and HDDB.This result indicates that the hydrogen transfer reaction in the RS isomer system is slightly more challenging than in the SS isomer system, consistent with the experimental results where the molecular weight of polypropylene obtained from the RS-containing catalyst is higher than that from the SS-containing catalyst.
Polymers 2024, 16, x 10 of 14 transfer reaction in the RS isomer system is slightly more challenging than in the SS isomer system, consistent with the experimental results where the molecular weight of polypropylene obtained from the RS-containing catalyst is higher than that from the SS-containing catalyst.The differences in catalytic activity and activation energy showed that the orientation for the methyl/ethyl groups of RS isomers led to the variations in hydrogen transfer reactions, which finally resulted in the distinct hydrogen response and polypropylene molecular weight for the catalysts.Nevertheless, due to the smaller space requirement of the hydrogen transfer reaction than the propylene insertion reaction, the variations in the difficulty of the hydrogen transfer reactions were relatively small.These results show that the variation trend for the activation energy of the hydrogen transfer reaction in different kinds of active centers is consistent with that of the propylene insertion reaction, with the variation range slightly different from each other.

Conclusions
We prepared ZN catalysts for propylene polymerization using the mixtures of racemate and mesomer isomers of HDDB as IDs.It was observed that the meso-isomers preferred to be adsorbed on the catalyst.As the relative content of meso-isomer increased, there was a corresponding rise in the total amount of IDs adsorbed on the catalysts.This  The differences in catalytic activity and activation energy showed that the orientation for the methyl/ethyl groups of RS isomers led to the variations in hydrogen transfer reactions, which finally resulted in the distinct hydrogen response and polypropylene molecular weight for the catalysts.Nevertheless, due to the smaller space requirement of the hydrogen transfer reaction than the propylene insertion reaction, the variations in the difficulty of the hydrogen transfer reactions were relatively small.These results show that the variation trend for the activation energy of the hydrogen transfer reaction in different kinds of active centers is consistent with that of the propylene insertion reaction, with the variation range slightly different from each other.S1.The polymerization results for the catalysts with different PDDB stereoisomers as ID.NMR, LC, GPC, and DFT calculation data (PDF).

Figure 1 .
Figure 1.The optimized DFT geometries for the HDDB stereoisomers in grey: carbon; hydrogen is omitted).

Table 2 .
Comparison between the calculated stereoselectivity for the tr insertion reaction of propylene, and the isotactic index obtained from the ID TS (re) − TS (si) a (kcal/mol) Calculated Stereoselectivity RS-PDDB 3SS-HDDB 1.1 82.7 a The data of PDDB have little difference to Ref. [49] because of the quasi-R free energy calculation.

Figure 1 .
Figure 1.The optimized DFT geometries for the HDDB stereoisomers in the free state (red: oxygen; grey: carbon; hydrogen is omitted).

Figure 1 .
Figure 1.The optimized DFT geometries for the HDDB stereoisomers in the free state (red: o grey: carbon; hydrogen is omitted).

Figure 5 .
Figure 5.The changes of macroscopic activity for the active centers with (a) PDDB and (b) HDDB ID when increasing the stereoisomer content.The residuals are distributed according to the relative content percent of the RS or SS stereoisomers.The slope represents the catalytic activity for the active centers per unit.

Figure 5 .
Figure 5.The changes of macroscopic activity for the active centers with (a) PDDB and (b) HDDB ID when increasing the stereoisomer content.The residuals are distributed according to the relative content percent of the RS or SS stereoisomers.The slope represents the catalytic activity for the active centers per unit.

Figure 6 .
Figure 6.The transition state structures of HDDB stereoisomer catalysts in the hydrogen transfer reaction (red: oxygen; grey: carbon; green: chlorine; yellow: magnesium; hydrogen is omitted).

Figure 6 .
Figure 6.The transition state structures of HDDB stereoisomer catalysts in the hydrogen transfer reaction (red: oxygen; grey: carbon; green: chlorine; yellow: magnesium; hydrogen is omitted).

Table 1 .
The polymerization results for the ZN catalysts with different HDDB stereoisomers as IDs.
a relative content of meso-HDDB in the same weight HDDB compound added in preparation of catalyst.b relative content of meso-HDDB in catalyst.c content of total HDDB (including mesomer and racemate) in catalyst.

Table 2 .
Comparison between the calculated stereoselectivity for the transition state in the si-/reinsertion reaction of propylene, and the isotactic index obtained from the experiments.

Table 3 .
Comparison between the calculated activation energy (kcal/mol) of si-insertion for the PDDB and HDDB isomers, and the calculated relative activity of SS/RS.

Table 4 .
The relative ratio of the activity regression values of individual active centers.